Standard Deviation

A normal distribution of data means that most of the examples in a set of data are close to the "average," while relatively few examples tend to one extreme or the other.

If you looked at normally distributed data on a graph, it would look something like this:

The standard deviation is a statistic that tells you how tightly all the various values are clustered around the mean in a set of data. 

The red area on the above graph is one standard deviation away from the mean in either direction on the horizontal axis and accounts for about 68% of the values in the set of data.  Two standard deviations away from the mean is shown by  the red and green areas and accounts for roughly 95 percent of the values. Three standard deviations (the red, green and yellow areas) account for about 99 percent of the values.